That does seem to be really large for an ePub. Most ePubs which contain hundreds of pages and a few images will be less than 4mb. The problem with PDFs is that they can contain text only, images of text only, or a combination. I've never had much luck converting a PDF that was mostly scanned images of text, and only rarely have I had luck converting one with text. PDFs and eBook readers really don't get along.

One thing you might do is to divide the document into two parts and create tw books instead on one so the sizes of the files are below 20mb.

Out of curiousity, did you try opening the PDF itself on the nookcolor?